1. Field of the Invention
The present invention relates to a polycarbonate copolymer and a method for producing the same. More particularly it relates to a polycarbonate copolymer which is superior in impact resistance, flame retardance, molding thermostability and the like, is good in hydrolytic resistance, is much less liable to cause a mold rust and further has high quality and a method for efficiently producing the same.
2. Description of Related Arts
As the halogen-containing copolymerized polycarbonate having high flame retardance, for example 1 a copolymer derived from tetrabromobisphenol A (TBA) and bisphenol A (BPA) (Japanese Patent Publication No. 24660/1972), 2 a copolymer derived from tetrabromobisphenolsulfone (TBS) and BPA (Japanese Patent Application Laid-Open No. 123294/1976), 3 a copolymer derived from tetrabromothiodiphenol (TBTDP) and BPA (Japanese Patent Application Laid-Open No. 99226/1981) and 4 a blend of the copolymer derived from a statistical mixture of halogenated bisphenol and BPA (Japanese Patent Application Laid-Open No. 136796/1976) or the copolymer derived from thiodiphenol (TDP) and BPA and the copolymer as set forth in 1 above (Japanese Patent Application Laid-Open No. 50065/1979) are known.
These copolymers contain a halogen in an amount necessary to have sufficiently high flame retardance through the copolymerization with halogenated bisphenols which are obtained by introducing halogen into the benzene nucleus of the bisphenols.
However, all of these polycarbonate copolymers have to have a comparatively high content of the halogenated bisphenols preventing a sufficiently satisfactory mechanical strength (particularly, impact resistance).
Other known examples of the halogen-containing polycarbonate copolymer are those in which a halogenated phenol is used as an end terminator (Japanese Patent Publication No. 40715/1971), but they are not sufficiently high in both flame retardance and mechanical strength.
Still other examples are the polycarbonates copolymerizing BPA, TBA and TDP to overcome the above shortcomings (Japanese Patent Application Laid-Open No. 140597/1977). Those polycarbonates are sufficiently high in the two of flame retardance and mechanical strength but not necessarily satisfactory with respect to moldability.
The study group of the present inventors have proposed a novel polycarbonate superior in flame retardance, mechanical strength and the like, in which a pentahalogenophenol is used as the end terminator and further BPA and TBA are copolymerized (Japanese Patent Application Laid-Open Nos. 79227/1989 and 79228/1989). This polycarbonate has been found to be excellent in flame retardance, mechanical strength and the like, but somewhat unsatisfactory in thermostability when the polycarbonate is molded.
In order to overcome the above problem, the study group of the present inventors have proposed a polycarbonate using a trihalogenophenol as the end terminator and copolymerizing BPA and TBA (Japanese Patent Application Laid-Open No. 200833/1991). It has been found, however, that something is missing in polycarbonates prepared by conventional polymerization techniques in connection with hydrolytic resistance and mold rust.
In view of the above problems of the related arts, the present inventors have made extensive studies with a view to developing a method for production of a polycarbonate which is superior in impact resistance, molding thermostability, color tone and the like, is of high hydrolytic resistance, is much less liable to cause mold rust and further has high quality. As the result, it has been found that the desired polycarbonate copolymer can be obtained according to a method which comprises steps of reacting bisphenol A with phosgene in the presence of an alkaline compound to react all of the phosgene in the reaction system into a polycarbonate oligomer; reacting the resultant polycarbonate oligomer with a tetrahalogenobisphenol A and a trihalogenophenol to prepare a polycarbonate copolymer oligomer; thereafter, separating the organic layer containing the resultant polycarbonate copolymer oligomer; and reacting the polycarbonate copolymer oligomer contained in the separated organic layer with bisphenol A and, if necessary, an end terminator in the presence of an alkaline compound. The resulting polycarbonate copolymer is superior in impact resistance, molding thermostability, color tone and the like, is good in hydrolytic resistance, is much less liable to cause mold rust and has high quality. The present invention has been completed on the basis of this finding.